#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using ll = long long;
using ul = unsigned long;
using ld = long double;
using st = string;
using mint = atcoder::modint998244353;
using Mint = atcoder::modint1000000007;
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vvvl vector<vector<vector<ll>>>
#define vvvvl vector<vector<vector<vector<ll>>>>
#define vd vector<ld>
#define vvd vector<vector<ld>>
#define vvvd vector<vector<vector<ld>>>
#define vb vector<bool>
#define vvb vector<vector<bool>>
#define vvvb vector<vector<vector<bool>>>
#define vs vector<string>
#define vvs vector<vector<string>>
#define vvvs vector<vector<vector<string>>>
#define vp vector<pair<ll,ll>>
#define vvp vector<vector<pair<ll,ll>>>
#define vvvp vector<vector<vector<pair<ll,ll>>>>
#define vm vector<mint>
#define vvm vector<vector<mint>>
#define vM vector<Mint>
#define vvM vector<vector<Mint>>
#define cmx(n,v) n=n<v?v:n
#define cmn(n,v) n=n>v?v:n
#define all(n) begin(n),end(n)
#define nxp(a) next_permutation(all(a))
#define rev(n) reverse(all(n))
#define sor(n) stable_sort(all(n))
#define rep(i,n) for(ll i=0;i<(n);++i)
#define reprev(i,n) for(ll i=n-1;0<=i;--i)
#define rrep(i,a,n) for(ll i=a;i<(n);++i)
#define sz(n) n.size()
#define bit(j,i) (i&(1<<j))
#define yn(x) cout << (x?"Yes":"No") << endl;
#define lb(vec,src) lower_bound(vec.begin(),vec.end(),src)-vec.begin()
#define ub(vec,src) upper_bound(vec.begin(),vec.end(),src)-vec.begin()
#define lb2(vec,src) *lower_bound(vec.begin(),vec.end(),src)
#define ub2(vec,src) *upper_bound(vec.begin(),vec.end(),src)
#define mne1(a) min_element(all(a))
#define mxe1(a) max_element(all(a))
#define mne2(a) *min_element(all(a))
#define mxe2(a) *max_element(all(a))
#define uniq(a) {sort(all(a)); (a).erase(unique(all(a)), (a).end());}
const ll inf = 9e18;

ll calc_digit(ll N){
    ll res = 0;
    while(N>0){
        res++;
        N/=10;
    }
    return res;
}

ll factorial(ll n, ll mod = 1e18) {
    ll ans = 1;
    for(ll i = n; i >= 2; i--) ans = (ans * i) % mod;
    return ans;
}

ll round_up(ll x,ll y){
    return (x+y-1)/y;
}

ll floor_int(ll x,ll m){
    ll r=(x%m+m)%m; //負の剰余を正の剰余に置き換える
    return (x-r)/m; //return
}

ll rmd(ll x,ll m){
    ll r=(x%m+m)%m; //負の剰余を正の剰余に置き換える
    return r;
}

ll rmd_2(ll x,ll m){
    ll r=(x%m+m)%m; //負の剰余を正の剰余に置き換える
    return (m-r)%m; //return
}

ll bubble_sort(vl a,ll n){
    ll res=0;
    rep(i,n-1){
        for(ll j=n-1;j>i;j--){
            if(a[j-1]>a[j]){
                swap(a[j-1],a[j]);
                res++;
            }
        }
    }
    return res;
}

ll sum_v(vl a){
    ll res=0;
    ll n=sz(a);
    rep(i,n) res+=a[i];
    return res;
}

ll stair_sum(ll from,ll to){
    return (from+to)*(to-from+1)/2;
}

ll Power(ll var, ll p) {
    if (p == 1) return var;
    ll ans = Power(var * var, p / 2);
    if (p & 1) ans *= var;;
    return ans;
}

ll cnt_01(ll n,ll now){
    ll res=0;
    res+=(n/now)*(now/2);
    if(n%now>=now/2) res+=n%now-(now/2)+1;
    return res;
}

ll sum_pop(ll n){
    ll res=0;
    for(ll b=0;b<60;b++){
        res+=(n/(1ll<<(b+1)))*(1ll<<b);
        res+=max(0ll,n%(1ll<<(b+1))-(1ll<<b)+1);
    }
    return res;
}

ll Tree_Diameter(vvl& g){
    ll n=sz(g);
    ll start=0;
    ll ans=0;
    for(ll i=0;i<2;i++){
        queue<ll> q;
        q.push(start);
        vl dis(n,inf);
        dis[start]=0;
        while(!q.empty()){
            ll u=q.front();
            q.pop();
            start=u;
            ans=dis[u];
            for(ll v:g[u]){
                if(dis[v]!=inf) continue;
                dis[v]=dis[u]+1;
                q.push(v);
            }
        }
    }
    return ans;
}

mint m_Power(mint var,ll p) {
    if (p == 1) return var;
    mint ans = m_Power(var * var, p / 2);
    if (p & 1) ans *= var;;
    return ans;
}

Mint M_Power(Mint var,ll p) {
    if (p == 1) return var;
    Mint ans = M_Power(var * var, p / 2);
    if (p & 1) ans *= var;;
    return ans;
}

bool is_prime(ll N){
    if(N==1) return false;
    if(N==2) return true;
    if(N%2==0) return false;
    for(ll p=3;p*p<=N;p+=2) if(N%p==0) return false;
    return true;
}

bool in_out(ll x,ll y,ll h,ll w){
    return 0<=x and x<h and 0<=y and y<w;
}

bool dis_solve1(ll x1,ll x2,ll y1,ll y2,ll r){
    ll xx=(x1-x2),yy=(y1-y2);
    ll dis=xx*xx+yy*yy;
    return (r*r==dis);
}

bool dis_solve2(ll x1,ll x2,ll y1,ll y2,ll d1,ll d2){
    ll xx=(x1-x2),yy=(y1-y2),r1=(d1+d2),r2=(d1-d2);
    ll dis=xx*xx+yy*yy;
    return (r2*r2<=dis&&dis<=r1*r1);
}

void printl(vl v,ll len=0){
    ll vsz=min(len,ll(v.size()));
    for(ll i=0;i<vsz;i++){
        cout << v[i] << endl;
    }
}

void prints(vs v){
    ll vsz=v.size();
    for(ll i=0;i<vsz;i++){
        cout << v[i] << endl;
    }
}

vl p_list(ll n){
    vl res;
    vb check(n+1);
    for(ll i=2;i<=n;i++){
        if(check[i]) continue;
        res.push_back(i);
        ll ii=i;
        while(ii<=n){
            check[ii]=true;
            ii+=i;
        }
    }
    return res;
}

vl cumulative_sum(vl a){
    ll s=sz(a);
    vl b(s+1,0);
    for(ll i=0;i<s;i++){
        b[i+1]=a[i]+b[i];
    }
    return b;
}

vl n_num(ll a,ll n){
    vl res;
    while(a){
        res.push_back(a%n);
        a/=n;
    }
    rev(res);
    return res;
}

vl c_press(vl a){
    ll n=sz(a);
    vl res=a,c=a;
    sor(c);
    uniq(c);
    rep(i,n) res[i]=lb(c,res[i]);
    return res;
}

vl min_fact(ll n){
    vl pl;
    vl res(n+1,-1);
    for(ll d=2;d<=n;d++){
        if(res[d]==-1){
            res[d]=d;
            pl.push_back(d);
        }
        for(ll p:pl){
            if(p*d>n||p>res[d]) break;
            res[p*d]=p;
        }
    }
    return res;
}

vvl to_grid(vl &a,ll k){
    ll n=sz(a);
    ll m=(n+k-1)/k;
    vvl b(k,vl(m,inf));
    rep(i,n) b[i%k][i/k]=a[i];
    return b;
}

vvl rotate_ll(vvl& V2d,ll wise){
    vvl a=V2d;
    ll h=sz(a),w=sz(a[0]);
    vvl res(w,vl(h));
    if(wise) {rep(i,h)rep(j,w) res[j][h-i-1]=a[i][j];}
    else {rep(i,h)rep(j,w) res[w-j-1][i]=a[i][j];}
    return res;
}

vs rotate_st(vs s,ll wise){
    vs a=s;
    ll h=sz(a),w=sz(a[0]);
    vs res(w);
    rep(i,w) res[i].append(h,'.');
    if(wise) {rep(i,h)rep(j,w) res[j][h-i-1]=a[i][j];}
    else {rep(i,h)rep(j,w) res[w-j-1][i]=a[i][j];}
    return res;
}

vp prime_factorize(ll N){
    vp res;
    for (ll a = 2; a * a <= N; ++a) {
        if (N % a != 0) continue;
        ll ex = 0;
        while (N % a == 0) {
            ex++;
            N /= a;
        }
        res.push_back({a, ex});
    }
    if (N != 1) res.push_back({N, 1});
    return res;
}

template <typename T> void input(T &a) { cin >> a; };
template <typename T1, typename... T2> void input(T1&a, T2 &...b) { cin >> a; input(b...); };

template<typename T = ll>
vector<T> rd(size_t n) {
  vector<T> ts(n);
  for (size_t i = 0; i < n; i++) cin >> ts[i];
  return ts;
}

template<typename TV, const ll N> void read_tuple_impl(TV&) {}
template<typename TV, const ll N, typename Head, typename... Tail>
void read_tuple_impl(TV& ts) {
  get<N>(ts).emplace_back(*(istream_iterator<Head>(cin)));
  read_tuple_impl<TV, N + 1, Tail...>(ts);
}
template<typename... Ts> decltype(auto) read_tuple(size_t n) {
  tuple<vector<Ts>...> ts;
  for (size_t i = 0; i < n; i++) read_tuple_impl<decltype(ts), 0, Ts...>(ts);
  return ts;
}

ll di[8]={1,-1,0,0,1,1,-1,-1};
ll dj[8]={0,0,1,-1,1,-1,-1,1};
//abcdefghijklmnopqrstuvwxyz
//std::setprecision(15);
//2^29<10^9

int main(){

	ios::sync_with_stdio(0);
	cin.tie(0);
    
    ll n;
    cin >> n;
    vl a=rd(n);
    vl cnt(3);

    rep(i,n){
        if(a[i]>=3) continue;
        cnt[a[i]]++;
    }

    map<array<ll,3>,ld> dp;
    dp[{0,0,0}]=0.0;

    auto e = [&] (auto f,ll i,ll j,ll k) -> ld {

        if(dp.count({i,j,k})) return dp[{i,j,k}];
        ld res=ld(n)/ld(i+j+k);

        if(i>0) res+=f(f,i-1,j+1,k)*(ld(i)/ld(i+j+k));
        if(j>0) res+=f(f,i,j-1,k+1)*(ld(j)/ld(i+j+k));
        if(k>0) res+=f(f,i,j,k-1)*(ld(k)/ld(i+j+k));
        return dp[{i,j,k}]=res;

    };

    ld ans=e(e,cnt[0],cnt[1],cnt[2]);
    cout << setprecision(15) << ans << endl;
    return 0;

}